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Add declval and common type from Vicente J. Botet Escriba.
Regenerate docs. [SVN r65443]
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John Maddock
parent
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doc/Jamfile.v2
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# Copyright John Maddock 2005. Use, modification, and distribution are
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# subject to the Boost Software License, Version 1.0. (See accompanying
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# file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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project : requirements
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# Path for links to Boost:
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<xsl:param>boost.root=../../../..
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# Some general style settings:
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<xsl:param>table.footnote.number.format=1
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<xsl:param>footnote.number.format=1
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# HTML options first:
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# Use graphics not text for navigation:
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<xsl:param>navig.graphics=1
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# PDF Options:
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# TOC Generation: this is needed for FOP-0.9 and later:
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<xsl:param>fop1.extensions=0
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<xsl:param>xep.extensions=1
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# TOC generation: this is needed for FOP 0.2, but must not be set to zero for FOP-0.9!
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<xsl:param>fop.extensions=0
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# No indent on body text:
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<xsl:param>body.start.indent=0pt
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# Margin size:
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<xsl:param>page.margin.inner=0.5in
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# Margin size:
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<xsl:param>page.margin.outer=0.5in
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# Paper type = A4
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<xsl:param>paper.type=A4
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<xsl:param>admon.graphics=1
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# default pnd graphics are awful in PDF form,
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# better use SVG's instead:
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<format>pdf:<xsl:param>admon.graphics.extension=".svg"
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<format>pdf:<xsl:param>admon.graphics.path=$(boost-images)/
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<format>pdf:<xsl:param>boost.url.prefix=http://www.boost.org/doc/libs/release/libs/utility/doc/html
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;
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using quickbook ;
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path-constant boost-images : ../../../doc/src/images ;
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xml declval : declval.qbk ;
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boostbook standalone
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:
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declval
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:
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# File name of HTML output:
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<xsl:param>root.filename=declval
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# How far down we chunk nested sections, basically all of them:
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<xsl:param>chunk.section.depth=0
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# Don't put the first section on the same page as the TOC:
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<xsl:param>chunk.first.sections=0
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# How far down sections get TOC's
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<xsl:param>toc.section.depth=1
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# Max depth in each TOC:
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<xsl:param>toc.max.depth=1
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# How far down we go with TOC's
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<xsl:param>generate.section.toc.level=1
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;
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104
doc/declval.qbk
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104
doc/declval.qbk
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[/
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/ Copyright (c) 2008 Howard Hinnant
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/ Copyright (c) 2008 Beman Dawes
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/ Copyright (c) 2009-20010 Vicente J. Botet Escriba
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/
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/ Distributed under the Boost Software License, Version 1.0. (See accompanying
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/ file LICENSE_1_0.txt or copy at http://www.boost.org/LICENSE_1_0.txt)
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/]
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[article Declval
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[quickbook 1.5]
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[authors [Hinnant, Howard]]
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[authors [Dawes, Beman]]
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[authors [Botet Escriba, Vicente J.]]
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[copyright 2008 Howard Hinnant]
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[copyright 2008 Beman Dawes]
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[copyright 2009-2010 Vicente J. Botet Escriba]
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[license
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Distributed under the Boost Software License, Version 1.0.
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(See accompanying file LICENSE_1_0.txt or copy at
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[@http://www.boost.org/LICENSE_1_0.txt])
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]
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]
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[/===============]
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[section Overview]
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[/===============]
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The motivation for `declval` was introduced in [@http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2958.html#Value N2958:
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Moving Swap Forward]. Here follows a rewording of this chapter.
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With the provision of decltype, late-specified return types, and default template-arguments for function templates a
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new generation of SFINAE patterns will emerge to at least partially compensate the lack of concepts on the C++0x timescale.
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Using this technique, it is sometimes necessary to obtain an object of a known type in a non-using context, e.g. given the declaration
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template<class T>
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T&& declval(); // not used
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as part of the function template declaration
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template<class To, class From>
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decltype(static_cast<To>(declval<From>())) convert(From&&);
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or as part of a class template definition
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template<class> class result_of;
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template<class Fn, class... ArgTypes>
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struct result_of<Fn(ArgTypes...)>
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{
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typedef decltype(declval<Fn>()(declval<ArgTypes>()...)) type;
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};
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The role of the function template declval() is a transformation of a type T into a value without using or evaluating this function.
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The name is supposed to direct the reader's attention to the fact that the expression `declval<T>()` is an lvalue if and only if
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T is an lvalue-reference, otherwise an rvalue. To extend the domain of this function we can do a bit better by changing its declaration to
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template<class T>
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typename std::add_rvalue_reference<T>::type declval(); // not used
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which ensures that we can also use cv void as template parameter. The careful reader might have noticed that `declval()`
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already exists under the name create() as part of the definition of the semantics of the type trait is_convertible in the C==0x standard.
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The provision of a new library component that allows the production of values in unevaluated expressions is considered as
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important to realize constrained templates in C++0x where concepts are not available.
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This extremely light-weight function is expected to be part of the daily tool-box of the C++0x programmer.
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[endsect]
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[/=================]
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[section:reference Reference ]
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[/=================]
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`#include <boost/utility/declval.hpp>`
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namespace boost {
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template <typename T>
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typename add_rvalue_reference<T>::type declval(); //noexcept; // as unevaluated operand
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} // namespace boost
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The library provides the function template declval to simplify the definition of expressions which occur as unevaluated operands.
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template <typename T>
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typename add_rvalue_reference<T>::type declval();
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[*Remarks:] If this function is used, the program is ill-formed.
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[*Remarks:] The template parameter T of declval may be an incomplete type.
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[*Example:]
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template <class To, class From>
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decltype(static_cast<To>(declval<From>())) convert(From&&);
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Declares a function template convert which only participats in overloading if the type From can be explicitly converted to type To.
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[endsect]
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163
doc/html/declval.html
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<html>
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<head>
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<meta http-equiv="Content-Type" content="text/html; charset=US-ASCII">
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<title>Declval</title>
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<link rel="stylesheet" href="../../../../doc/src/boostbook.css" type="text/css">
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<meta name="generator" content="DocBook XSL Stylesheets V1.74.0">
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<link rel="home" href="declval.html" title="Declval">
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</head>
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<body bgcolor="white" text="black" link="#0000FF" vlink="#840084" alink="#0000FF">
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<table cellpadding="2" width="100%"><tr>
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<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86" src="../../../../boost.png"></td>
|
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<td align="center"><a href="../../../../index.html">Home</a></td>
|
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<td align="center"><a href="../../../../libs/libraries.htm">Libraries</a></td>
|
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<td align="center"><a href="http://www.boost.org/users/people.html">People</a></td>
|
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<td align="center"><a href="http://www.boost.org/users/faq.html">FAQ</a></td>
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<td align="center"><a href="../../../../more/index.htm">More</a></td>
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</tr></table>
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<hr>
|
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<div class="spirit-nav"></div>
|
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<div class="article" lang="en">
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<div class="titlepage">
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<div>
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<div><h2 class="title">
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<a name="declval"></a>Declval</h2></div>
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<div><div class="authorgroup">
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<div class="author"><h3 class="author">
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<span class="firstname">Howard</span> <span class="surname">Hinnant</span>
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</h3></div>
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<div class="author"><h3 class="author">
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<span class="firstname">Beman</span> <span class="surname">Dawes</span>
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</h3></div>
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<div class="author"><h3 class="author">
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<span class="firstname">Vicente J.</span> <span class="surname">Botet Escriba</span>
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</h3></div>
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</div></div>
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<div><p class="copyright">Copyright © 2008 Howard Hinnant</p></div>
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<div><p class="copyright">Copyright © 2008 Beman Dawes</p></div>
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<div><p class="copyright">Copyright © 2009 -2010 Vicente J. Botet Escriba</p></div>
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<div><div class="legalnotice">
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<a name="id879409"></a><p>
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Distributed under the Boost Software License, Version 1.0. (See accompanying
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file LICENSE_1_0.txt or copy at <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
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</p>
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</div></div>
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</div>
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<hr>
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</div>
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<div class="toc">
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<p><b>Table of Contents</b></p>
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<dl>
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<dt><span class="section"><a href="declval.html#declval.overview">Overview</a></span></dt>
|
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<dt><span class="section"><a href="declval.html#declval.reference"> Reference </a></span></dt>
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</dl>
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</div>
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<div class="section" lang="en">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
|
||||
<a name="declval.overview"></a><a class="link" href="declval.html#declval.overview" title="Overview">Overview</a>
|
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</h2></div></div></div>
|
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<p>
|
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The motivation for <code class="computeroutput"><span class="identifier">declval</span></code>
|
||||
was introduced in <a href="http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2009/n2958.html#Value" target="_top">N2958:
|
||||
Moving Swap Forward</a>. Here follows a rewording of this chapter.
|
||||
</p>
|
||||
<p>
|
||||
With the provision of decltype, late-specified return types, and default template-arguments
|
||||
for function templates a new generation of SFINAE patterns will emerge to at
|
||||
least partially compensate the lack of concepts on the C++0x timescale. Using
|
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this technique, it is sometimes necessary to obtain an object of a known type
|
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in a non-using context, e.g. given the declaration
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</p>
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<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
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<span class="identifier">T</span><span class="special">&&</span> <span class="identifier">declval</span><span class="special">();</span> <span class="comment">// not used
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</span></pre>
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<p>
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as part of the function template declaration
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</p>
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<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">To</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">From</span><span class="special">></span>
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<span class="identifier">decltype</span><span class="special">(</span><span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">To</span><span class="special">>(</span><span class="identifier">declval</span><span class="special"><</span><span class="identifier">From</span><span class="special">>()))</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">From</span><span class="special">&&);</span>
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</pre>
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<p>
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or as part of a class template definition
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</p>
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<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span><span class="special">></span> <span class="keyword">class</span> <span class="identifier">result_of</span><span class="special">;</span>
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<span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">Fn</span><span class="special">,</span> <span class="keyword">class</span><span class="special">...</span> <span class="identifier">ArgTypes</span><span class="special">></span>
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<span class="keyword">struct</span> <span class="identifier">result_of</span><span class="special"><</span><span class="identifier">Fn</span><span class="special">(</span><span class="identifier">ArgTypes</span><span class="special">...)></span>
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<span class="special">{</span>
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<span class="keyword">typedef</span> <span class="identifier">decltype</span><span class="special">(</span><span class="identifier">declval</span><span class="special"><</span><span class="identifier">Fn</span><span class="special">>()(</span><span class="identifier">declval</span><span class="special"><</span><span class="identifier">ArgTypes</span><span class="special">>()...))</span> <span class="identifier">type</span><span class="special">;</span>
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<span class="special">};</span>
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</pre>
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<p>
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The role of the function template declval() is a transformation of a type T
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into a value without using or evaluating this function. The name is supposed
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to direct the reader's attention to the fact that the expression <code class="computeroutput"><span class="identifier">declval</span><span class="special"><</span><span class="identifier">T</span><span class="special">>()</span></code> is
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an lvalue if and only if T is an lvalue-reference, otherwise an rvalue. To
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extend the domain of this function we can do a bit better by changing its declaration
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to
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</p>
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<pre class="programlisting"><span class="keyword">template</span><span class="special"><</span><span class="keyword">class</span> <span class="identifier">T</span><span class="special">></span>
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<span class="keyword">typename</span> <span class="identifier">std</span><span class="special">::</span><span class="identifier">add_rvalue_reference</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">declval</span><span class="special">();</span> <span class="comment">// not used
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</span></pre>
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<p>
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which ensures that we can also use cv void as template parameter. The careful
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reader might have noticed that <code class="computeroutput"><span class="identifier">declval</span><span class="special">()</span></code> already exists under the name create() as
|
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part of the definition of the semantics of the type trait is_convertible in
|
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the C==0x standard.
|
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</p>
|
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<p>
|
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The provision of a new library component that allows the production of values
|
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in unevaluated expressions is considered as important to realize constrained
|
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templates in C++0x where concepts are not available. This extremely light-weight
|
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function is expected to be part of the daily tool-box of the C++0x programmer.
|
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</p>
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</div>
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<div class="section" lang="en">
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<div class="titlepage"><div><div><h2 class="title" style="clear: both">
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<a name="declval.reference"></a><a class="link" href="declval.html#declval.reference" title="Reference"> Reference </a>
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</h2></div></div></div>
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<p>
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<code class="computeroutput"><span class="preprocessor">#include</span> <span class="special"><</span><span class="identifier">boost</span><span class="special">/</span><span class="identifier">utility</span><span class="special">/</span><span class="identifier">declval</span><span class="special">.</span><span class="identifier">hpp</span><span class="special">></span></code>
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</p>
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<pre class="programlisting"><span class="keyword">namespace</span> <span class="identifier">boost</span> <span class="special">{</span>
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<span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
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<span class="keyword">typename</span> <span class="identifier">add_rvalue_reference</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">declval</span><span class="special">();</span> <span class="comment">//noexcept; // as unevaluated operand
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</span>
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<span class="special">}</span> <span class="comment">// namespace boost
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</span></pre>
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<p>
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The library provides the function template declval to simplify the definition
|
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of expressions which occur as unevaluated operands.
|
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</p>
|
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<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">typename</span> <span class="identifier">T</span><span class="special">></span>
|
||||
<span class="keyword">typename</span> <span class="identifier">add_rvalue_reference</span><span class="special"><</span><span class="identifier">T</span><span class="special">>::</span><span class="identifier">type</span> <span class="identifier">declval</span><span class="special">();</span>
|
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</pre>
|
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<p>
|
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<span class="bold"><strong>Remarks:</strong></span> If this function is used, the program
|
||||
is ill-formed.
|
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</p>
|
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<p>
|
||||
<span class="bold"><strong>Remarks:</strong></span> The template parameter T of declval
|
||||
may be an incomplete type.
|
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</p>
|
||||
<p>
|
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<span class="bold"><strong>Example:</strong></span>
|
||||
</p>
|
||||
<pre class="programlisting"><span class="keyword">template</span> <span class="special"><</span><span class="keyword">class</span> <span class="identifier">To</span><span class="special">,</span> <span class="keyword">class</span> <span class="identifier">From</span><span class="special">></span>
|
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<span class="identifier">decltype</span><span class="special">(</span><span class="keyword">static_cast</span><span class="special"><</span><span class="identifier">To</span><span class="special">>(</span><span class="identifier">declval</span><span class="special"><</span><span class="identifier">From</span><span class="special">>()))</span> <span class="identifier">convert</span><span class="special">(</span><span class="identifier">From</span><span class="special">&&);</span>
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</pre>
|
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<p>
|
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Declares a function template convert which only participats in overloading
|
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if the type From can be explicitly converted to type To.
|
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</p>
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</div>
|
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</div>
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<table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
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<td align="left"><p><small>Last revised: September 16, 2010 at 16:19:10 GMT</small></p></td>
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||||
<td align="right"><div class="copyright-footer"></div></td>
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||||
</tr></table>
|
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<hr>
|
||||
<div class="spirit-nav"></div>
|
||||
</body>
|
||||
</html>
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44
include/boost/utility/declval.hpp
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44
include/boost/utility/declval.hpp
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// common_type.hpp ---------------------------------------------------------//
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|
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// Copyright 2010 Vicente J. Botet Escriba
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||||
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// Distributed under the Boost Software License, Version 1.0.
|
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// See http://www.boost.org/LICENSE_1_0.txt
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#ifndef BOOST_TYPE_TRAITS_EXT_DECLVAL__HPP
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#define BOOST_TYPE_TRAITS_EXT_DECLVAL__HPP
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#include <boost/config.hpp>
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//----------------------------------------------------------------------------//
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#include <boost/type_traits/add_rvalue_reference.hpp>
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|
||||
//----------------------------------------------------------------------------//
|
||||
// //
|
||||
// C++03 implementation of //
|
||||
// Written by Vicente J. Botet Escriba //
|
||||
//~ 20.3.4 Function template declval [declval]
|
||||
//~ 1 The library provides the function template declval to simplify the definition of expressions which occur as
|
||||
//~ unevaluated operands.
|
||||
//~ 2 Remarks: If this function is used, the program is ill-formed.
|
||||
//~ 3 Remarks: The template parameter T of declval may be an incomplete type.
|
||||
//~ [ Example:
|
||||
|
||||
//~ template <class To, class From>
|
||||
//~ decltype(static_cast<To>(declval<From>())) convert(From&&);
|
||||
|
||||
//~ declares a function template convert which only participats in overloading if the type From can be
|
||||
//~ explicitly converted to type To. For another example see class template common_type (20.7.6.6). —end
|
||||
//~ example ]
|
||||
// //
|
||||
//----------------------------------------------------------------------------//
|
||||
|
||||
namespace boost {
|
||||
|
||||
template <typename T>
|
||||
typename add_rvalue_reference<T>::type declval(); //noexcept; // as unevaluated operand
|
||||
|
||||
} // namespace boost
|
||||
|
||||
#endif // BOOST_TYPE_TRAITS_EXT_DECLVAL__HPP
|
||||
@ -22,6 +22,7 @@
|
||||
<a href="checked_delete.html">checked_delete</a><br>
|
||||
<a href="compressed_pair.htm">compressed_pair</a><br>
|
||||
<a href="current_function.html">current_function</a><br>
|
||||
<a href="doc/html/declval.html">declval</a><br>
|
||||
<a href="enable_if.html">enable_if</a><br>
|
||||
<a href="in_place_factories.html">in_place_factory</a><br>
|
||||
<a href="iterator_adaptors.htm">iterator_adaptors</a><br>
|
||||
@ -33,7 +34,8 @@
|
||||
<a href="swap.html">swap</a><br>
|
||||
<a href="throw_exception.html">throw_exception</a><br>
|
||||
<a href="utility.htm">utility</a><br>
|
||||
<a href="value_init.htm">value_init</a></p>
|
||||
<a href="value_init.htm">value_init</a>
|
||||
</p>
|
||||
</blockquote>
|
||||
<hr>
|
||||
<p>© Copyright Beman Dawes, 2001</p>
|
||||
|
||||
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Reference in New Issue
Block a user